The selection of appropriate containers and processes for packaging a particular liquid food product presents the packer with various options from which to choose. Against these must be considered numerous factors. These include cost of the container and packaging process, structural integrity and weight of the package, handling convenience for the packer and convenience for the consumer i.e., can the container be tightly packed? Is it fragile? The packer must also consider the distribution network and infrastructure available in the market sought to be served: is refrigeration available, if not, the packaged product must be rendered shelf stable?
A package which is too costly in relation to the value of the product itself or which is inconvenient in some respect for the consumer will meet with market resistance and likely fail to win a sufficient degree of acceptance to be commercially viable.
Traditionally juices have been packed in cans and glass bottles. The technology applicable to these packages is well known. Juices packed in these containers may be kept unrefrigerated for extended periods provided that these products and their containers have been sterilized and hermetically sealed in order To prevent entry of bacteria.
These containers have achieved wide acceptance through use. They may be easily sterilized and sealed effectively. They tolerate both internal vacuum and positive pressure. Closures such a s twist off lids for bottles are acceptable to consumers-because they are easy to operate and may be used to close the bottles effectively after first opening. These containers resist high temperatures and thus may be sterilized at usual temperatures without deteriorating their structural qualities.
Among the disadvantages of glass bottles and cans are cost and the weight which they add to the product. Also, because of their shape, cans and bottles cannot be tightly packed. In the case of bottles their relative fragility is also a disadvantage.
More recently greater use of so-called flexible containers has been made for the packaging of liquid food products. These include paper-based containers, molded plastic containers and plastic pouches.
Two types of paper-based flexible containers have become widely accepted, the gable top carton and the brick pack. The gable top carton has been extensively used for packaging refrigerated non-shelf stable liquid products such as milk and other dairy products and fruit juice products. The brick pack, so called because of its shape, has been extensively used in association with an aseptic packing process for fruit juices packaged for shelf stability and which therefore do not require refrigeration.
The gable top carton is relatively inexpensive and light weight. Moreover, because of its planar sides it can be tightly packed for shipping and storage. The gable top carton enjoys wide acceptance in the public for its ease of storage and handling after opening and for its spout which pours well and re-closes effectively even to the extent of permitting the contents to be lightly shaken without spilling. One disadvantage of the gable top carton, when compared with glass bottles, is the difficulty encountered by many consumers in opening the carton for the first time. This problem has been largely overcome in the dairy and fruit juice industry by simply weakening the seal at the spout.
A previously proposed method of weakening the spout is described in U.S. Pat. No. 3,116,002, wherein it is suggested that a non-adherent material "adhesive" be applied to selected areas near the spout in order to reduce the plastic-to-plastic bond formed during the sealing process. This method adds to the cost of the carton and quality control as to the precise placement of the adhesive is difficult to achieve during packing.
While the otherwise desirable goal of achieving a strong hermetic seal is defeated the consumer has become accustomed to the fact that dairy products and juices packed in this way must be kept refrigerated and have only limited shelf lives. Because of this practice of weakening the seal to please the consumer, from a consumer acceptance point of view, it is not possible to successfully market a juice in a gable top carton unless the spout is relatively easy to open.
Unfortunately, in some ways the gable top carton lacks the structural integrity of other containers, especially cans. For instance, gable top cartons are relatively intolerant of internal vacuum or pressure. Also their physical properties make them less tolerant of high sterilization temperatures. As discussed below, these drawbacks have inhibited use of gable top cartons in shelf stable packaging.
Typically, gable top cartons are formed of paperboard sheet material having an overall coating of thermoplastic film such as polyethylene applied on the surfaces of the sheet. The plastic coated paperboard is impervious to moisture and is particularly suitable for use in packaging products such as milk and orange juice. The thermoplastic coating not only serves to moisture proof the carton but also can be utilized for sealing the carton closing flaps which characterize the gable too carton. This sealing action is accomplished by heating the surfaces to be sealed while pressing them tightly together to form a liquid tight plastic-to-plastic bond.
The carton erected from the plastic-coated blank typically includes a bottom, four sidewalls extending from the bottom and united along their lateral margins, and an upper closure means which can be opened to form a spout. The closure is formed from four end wall portions united with the sidewalls and with each other at the lateral margins; the end wall portions are inclined in pairs toward each other and have marginal portions heat sealed to each other which form extensions of the end wall portions; the marginal portions of two opposite end wall portions are folded like bellows between the outer marginal portions of the two remaining end wall portions and at least one of the two inwardly folded marginal portions is moveable outward together with the pertaining end wall portions from its position between the two outer marginal portions to a new position outside these where the marginal portion after being moved outward, as well as its pertaining end wall portion, form a pouring spout and an emptying passage.
Traditionally, dairy products and fruit juices packed in gable top cartons have been kept refrigerated throughout the distribution process in order to avoid almost immediate spoilage. This spoilage would result from the fact that the gable top cartons are not sterilized and that bacteria in the environment and on the inside of the carton itself would develop very quickly. It is only refrigeration which retards spoilage for up to a couple of weeks. Products packed in this way are not said to be shelf stable.
Shelf stability is extremely desirable from many standpoints. A shelf stable product is much less likely to spoil while in the distribution system and with a shelf life measured in months rather than days losses due to spoilage should be low. The packer does not need to maintain the product under refrigeration either in its warehouse or while in transport. Similarly the retailer need not allocate expensive refrigerated space to store its supply of product. The consumer also has the advantage of a product which does not require refrigeration until opened.
Shelf stability may only be achieved in two ways. First, use may be made of chemical preservatives. Alternatively, precautions must be taken to exclude the possibility of bacteria coming into contact with the product following pasteurization of the product itself.
In order to achieve shelf stability of a juice or similar liquid food product without using chemical preservatives the product must be sterilized (said to be pasteurized). Following pasteurization no bacteria must be permitted to come into contact with the product. To ensure that this is the case, all bacteria which may come into contact with the product prior to or at the time of filling and sealing of the carton, all bacteria within the carton and all bacteria in the environment which become trapped in the carton after sealing must be killed. Failure to do so will inevitably lead to spoilage.
With glass containers and cans a process known as post packaging sterilization has been employed to achieve shelf stability. The liquid is poured into a non-sterile container, the container is capped and sealed; the container and its contents are heated to and maintained at a sterilization temperature until all the bacteria within the container are killed; the container is then cooled; the product is then shelf stable.
This process has been used in the brewing and soft drink trade in packaging liquids in bottles and cans. It has also been used to package liquids in flexible containers. For example, U.S. Pat. Nos. 4,088,444 to Byrne and 2,995,418 to Muller each disclose a sterilization process wherein a container formed from flexible material such as plastic is filled with a liquid, sealed, and sterilized by first subjecting the container to a hot liquid bath or to steam until the temperature of the liquid in the containers reaches the sterilization temperature, maintaining the temperature at the sterilization temperature until sterilization is effected, and thereafter cooling the liquid in the containers by subjecting the containers to cold water. Muller in particular specifically contemplates sterilizing fruit juices which are in sealed plastic containers.
U.S. Pat. No. 4,376,126 to Evers discloses a method of making a yogurt beverage which includes the steps of pasteurization or sterilization after the beverage has been packed in a container. Evers, however, does not appear to specifically disclose the use of any particular container nor does he mention the special problems associated with gable top cartons.
U.S. Pat. Nos. 4,057,391 to Yamaguchi, 3,481,688 to Craig et al. and 2,380,134 to Waters disclose methods of preserving or sterilizing food in flexible containers, such as plastic containers, whereby the food is subjected to heating and then cooling after being sealed in the containers. However, these patents do not specifically teach the sterilization of liquid food products, such as fruit juices nor do they address the special problems associated with gable top cartons.
U.S. Pat. No. 237,449 to Schaumberg et al. discloses a method of preserving fruit juices in glass bottles that includes bottling and sealing the juices and thereafter heating the juices to 170.degree. F. for 35 minutes. However, Schaumberg et al. does not disclose subjecting the heated bottles to a cooling process and does not contemplate the use of plastic or plastic coated cartons nor does he address the special problems of gable cartons.
Despite the wide recognition of the usefulness of most package sterilization in other packaging systems, the process has not to the inventor's knowledge previously been used to package perishable liquids in gable top cartons.
Since the introduction in the 1950's of paper-based cartons for liquid food products, only two processes have been widely promoted in order to permit paper-based cartons to be used for packaging shelf stable products. These are the aseptic process, promoted since the early 1960's by Tetra Pak of Sweden principally in association with the so-called brick pack, and the hot fill process more recently adapted for gable top cartons from a process used in connection with glass bottles.
In accordance with the aseptic process the product is pasteurized and kept in a sterile environment up to and through the filling and sealing of the container which is pre-sterilized (generally using hydrogen peroxide and steam) and also kept in a sterile environment until filling and sealing. The seal must be hermetic under all conditions. Aseptically processed brick packs must be cut, pierced or torn to open. In each case either the opening or the re-closing is clumsy and is not considered advantageous from a consumer convenience point of view. The aseptic method is complicated to carry out and requires specialized and expensive equipment.
Notwithstanding the complexity and cost of the aseptic process it became widely accepted in Europe, Australia and Canada in the 1970's. It was only approved in the United States in the early 1980's and has become widely accepted since that time. Prior to the introduction of the aseptic process and the brick pack there were no shelf stable products packaged in paper-based containers. The aseptically packed brick pack created a whole new application trot paper-based containers. The success of the aseptically packed brick pack has been nothing short of phenomenal. It is estimated that, on a worldwide basis, aseptically packed brick packs account for over $20 billion dollars worth of liquid food products.
Because of the complexity and cost of the aseptic process, efforts were made to achieve shelf stability in paper-based containers, most desirably with the popular gable top carton, with simpler less expensive technology. In the early 1980's a so called hot fill process was proposed.
A less costly alternative to the aseptic method, the hot fill method did not meet with much success. In accordance with the hot fill method the pasteurized product is heated to temperatures in the range of 185 degrees F. and poured hot directly into the gable top carton which is then sealed. The heat of the product itself kills the bacteria remaining within the container after sealing.
In the United States the hot fill method for gable cartons was adopted by various juice packers but has largely been abandoned because this method results in a substantial vacuum being created within the container following the cooling of the product from the hot fill temperature to room temperature. This vacuum is undesirable because it promotes the inflow of bacteria from the environment through the seal which leads to quick spoilage unless the product is refrigerated. In order to deal with this problem the seal must be strong enough to withstand the significant pressure differential created by the process, The seal must also be strong enough to retain its integrity in all cases throughout the distribution process. If at any time seal integrity is lost, the vacuum will cause bacteria to enter the carton and virtually immediate spoilage to occur.
Where the seal was made sufficiently strong to prevent the entry of bacteria under all conditions, the cartons were so hard to open that consumers, used to the ease of opening of the weakly sealed gable cartons used for dairy products, resisted the container.
In practice, the strength of the seal which had to be provided to overcome the problems created by the high vacuum resulted in a significant percentage of the cartons not being readily openable by the consumer in the ordinary way. This was such a problem that International Paper, the principal proponent of the hot fill technology, developed an alternative form of opening the carton, subject of U.S. Pat. No. 4,527,732 issued in 1985, which covers a scored "smile" to be pushed through by the consumer.
Thus up until the present invention industry efforts to adapt the popular gable top carton to achieve shelf stability and yet retain its good opening and handling qualities all at reasonable cost have heretofore not proven successful; and this despite the fact that finding an uncomplicated and inexpensive method of achieving shelf stability with a gable top carton would open up a market worth at least several hundred million dollars for cartons alone.
Thus, there remains a need for a system for shelf stable packaging of perishable liquids which allows use of a gable top carton which can be hermetically sealed to ensure shelf stability while retaining an easily opened spout necessary for consumer acceptance.